The present invention relates to an efficient self-oscillating burst mode transmitter or oscillator used with implantable devices, such as medical devices. More particularly, the present invention relates to a self-oscillating burst mode transmitter or oscillator wherein each burst of the transmitted or generated output signal contains an integral number of periods of a carrier signal at a substantially constant amplitude. The invention finds primary application in communicating with an implantable medical device, such as a pacemaker.
Burst mode transmitters or oscillators transmit or generate a carrier signal for a short period of time, termed the "burst" time, and do not transmit or generate the carrier signal in between burst times, or "bursts." Such transmitters or oscillators are commonly used in communication channels, e.g., where digital data is transferred using an ON-OFF keyed encoding scheme. In accordance with an ON-OFF keyed transmission, the presence of a carrier signal indicates one binary value, such as a binary "1", and the absence of the carrier signal indicates the other binary value, such as a binary "0". Thus, a burst mode transmitter may be used to transmit one binary value through the transmission of a burst, and to transmit the other binary value through the absence of a burst.
In general, burst mode transmitters or oscillators may be classified as: (1) self-oscillating, in which case the carrier signal is generated by the transmitter or oscillator upon application of power thereto; or (2) clock-driven, in which case an external clock signal provides the basic signal from which the carrier signal is derived. In general, the self-oscillating type of transmitter or oscillator may operate in two modes. A first mode is a continuous mode, in which case the carrier signal is generated continuously (100% duty cycle), with a burst of the carrier signal being obtained by gating the carrier signal for the desired burst time, and blocking it in between burst times. This gated signal is then applied to a transmitting coil (or other load). A second mode is a burst mode, in which case the carrier signal is generated only during the desired burst time period (less than 100% duty cycle), and is not generated during non-burst time periods. Clock-driven oscillators, as defined above, always operate in the continuous mode because the basic clock signal is always present at a 100% duty cycle.
Continuous mode operation of a burst mode transmitter or oscillator is inefficient. That is, a significant amount of power may be consumed during the non-burst periods when operation of the transmitter or oscillator is not needed. The ideal burst mode transmitter or oscillator, from a power efficient stand point, is ON only when transmitting or generating a burst of the carrier signal, and OFF at all other times.
Unfortunately, even when operating in a burst mode, a self-oscillating burst mode transmitter is generally not able to operate at maximum efficiency. This is because the carrier signal must typically be applied to an inductive load (transmitting coil or antenna), and the transient response associated with suddenly applying a signal to an inductive load prevents the applied carrier signal from assuming its maximum value (amplitude) across the inductive load throughout the entire duration of the transmitted burst. Hence, a portion of each burst, typically more than one cycle of the carrier signal, must usually be transmitted at less than maximum power. While this effect can be minimized by increasing the duration of the burst, such increase of burst time may significantly slow down the rate at which data can be transferred (the burst rate). Further, while this effect can also be minimized in some instances by increasing the frequency of the carrier signal, and thereby increasing the number of cycles of the carrier signal within each burst, it is often undesirable or not possible to increase the carrier signal frequency due to the bandwidth limitations of the available communications channel. What is needed, therefore, is a self-oscillating burst mode transmitter that provides for the efficient transmission of a carrier signal throughout the entire duration of the transmitted burst.
It is known in the art to use a tank circuit in a self-oscillating burst mode oscillator or transmitter for the purpose of generating the carrier frequency. The tank circuit consists of an inductor and a capacitor connected in a suitable series or parallel circuit configuration. When the tank circuit is energized with power of an appropriate polarity, the tank circuit begins to resonate at its resonant frequency. (The resonant frequency of a tank circuit is a function of the inductor and capacitor values used therein.) If power of an appropriate polarity is periodically applied to the tank circuit, the oscillations at the resonant frequency continue. Unfortunately, it typically takes several cycles of the resonating carrier signal before a steady-state oscillatory condition exists within the tank circuit. Much of the energy pumped into the tank circuit in order to achieve this steady-state condition is lost if the tank circuit is suddenly started, as occurs at the beginning of a transmitted burst, or if the tank circuit is suddenly shut down, as occurs at the end of a transmitted burst. Thus, more efficient operation of the transmitter or oscillator circuit could be achieved if the energy in the tank circuit at the conclusion of a burst could be saved and used at the beginning of the next burst.
Further, when operating a transmitter in a burst mode, it may be desirable to control the exact number of cycles of the carrier signal that occur during each transmitted burst. Heretofore, the number of cycles appearing in a given burst has been only approximately determined by controlling the duration of the transmitted burst from an external time base. What is needed is a more precise technique for controlling the number of cycles of the carrier signal appearing in each burst, and more particularly a technique that accurately places a programmable integral number of cycles of the carrier signal in each burst.
The present invention advantageously addresses the above and other needs.